They could do something like Crimson Tempest causes Venomous Wounds allowing us to FoK more and just use CT and envenom insted of having to put up rupture, although i dont think our aoe is that bad, and this might be op.

To look at the ramifications of the concept, since FoK presumably still isn't a finisher, you'd hit 5 CP, replace rupture with FoK on single target and continue your rotation. This would be roughly equivalent to FoK generating 6 CP instead of 1 CP.

As others have said, making CT useful (for assassination) would go a long ways further than changing FoK more. The ability's already there... it just hits like a wet noodle. Making it proc a (weaker, 0 energy return) VW would be the best easy way to buff it if Blizzard decided assassination's AoE was lacking.

I like that assassination doesn't use CT. Each spec has its own unique AoE rotation, they aren't all that different but it is one of the places Blizzard has done a good job with spec individuality although possibly by accident.

Combat puts up BF does a single target rotation replacing Evis with CT if the bleed isn't up.
Sub spams FoK finishing with CT.
Assassination uses FoK as CP builder and either multi-dots or does a single target rotation.